diff --git a/apps/backend/internal/github/mock_client.go b/apps/backend/internal/github/mock_client.go
index 15920a067b..6f0e8a7fa6 100644
--- a/apps/backend/internal/github/mock_client.go
+++ b/apps/backend/internal/github/mock_client.go
@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"time"
 )
 
@@ -77,6 +78,18 @@ type MockClient struct {
 	gists            map[string]mockGist
 	deletedGists     []string
 	nextGistID       int
+
+	// findPRByBranchCalls counts FindPRByBranch invocations so tests can
+	// assert that branch-detection probes are throttled. Atomic because
+	// FindPRByBranch otherwise only takes a read lock.
+	findPRByBranchCalls atomic.Int64
+
+	// probeEntered/probeRelease let a test gate FindPRByBranch: when set, each
+	// invocation signals on probeEntered and then blocks until probeRelease is
+	// closed. Used to force concurrent probes to overlap and assert
+	// singleflight coalescing.
+	probeEntered chan string
+	probeRelease chan struct{}
 }
 
 // mockGist captures a gist that was created via the mock client so tests
@@ -133,12 +146,38 @@ func (m *MockClient) GetPR(_ context.Context, owner, repo string, number int) (*
 }
 
 func (m *MockClient) FindPRByBranch(_ context.Context, owner, repo, branch string) (*PR, error) {
+	m.findPRByBranchCalls.Add(1)
 	m.mu.RLock()
-	defer m.mu.RUnlock()
 	pr := m.prsByBranch[branchKey{owner, repo, branch}]
+	entered, release := m.probeEntered, m.probeRelease
+	m.mu.RUnlock()
+	// Gate (if a test installed one) outside the lock so a blocked probe
+	// doesn't wedge other mock operations.
+	if entered != nil {
+		entered <- branch
+	}
+	if release != nil {
+		<-release
+	}
 	return pr, nil
 }
 
+// FindPRByBranchCallCount returns how many times FindPRByBranch has been
+// called. Used by tests asserting detection-probe throttling.
+func (m *MockClient) FindPRByBranchCallCount() int {
+	return int(m.findPRByBranchCalls.Load())
+}
+
+// GateFindPRByBranch installs a gate around FindPRByBranch: each invocation
+// signals on entered, then blocks until release is closed. Pass nil/nil to
+// remove the gate. Used to force concurrent probes to overlap.
+func (m *MockClient) GateFindPRByBranch(entered chan string, release chan struct{}) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	m.probeEntered = entered
+	m.probeRelease = release
+}
+
 func (m *MockClient) ListAuthoredPRs(_ context.Context, owner, repo string) ([]*PR, error) {
 	m.mu.RLock()
 	defer m.mu.RUnlock()
@@ -527,6 +566,9 @@ func (m *MockClient) Reset() {
 	m.gists = make(map[string]mockGist)
 	m.deletedGists = nil
 	m.nextGistID = 0
+	m.findPRByBranchCalls.Store(0)
+	m.probeEntered = nil
+	m.probeRelease = nil
 }
 
 // SubmittedReviews returns all recorded SubmitReview calls.
diff --git a/apps/backend/internal/github/service_pr_sync_test.go b/apps/backend/internal/github/service_pr_sync_test.go
index d1be09f36e..31420f5841 100644
--- a/apps/backend/internal/github/service_pr_sync_test.go
+++ b/apps/backend/internal/github/service_pr_sync_test.go
@@ -2,7 +2,9 @@ package github
 
 import (
 	"context"
+	"sync"
 	"testing"
+	"testing/synctest"
 )
 
 func TestTriggerPRSync_SyncsExistingWatch(t *testing.T) {
@@ -108,3 +110,102 @@ func TestTriggerPRSync_NoWatch(t *testing.T) {
 		t.Errorf("expected nil TaskPR for task without watch, got %+v", result)
 	}
 }
+
+// TestTriggerPRSyncAll_ThrottlesDetectionProbe reproduces the log-flood bug:
+// a pr_number=0 watch on a branch that has no PR (e.g. an unresolvable repo)
+// was re-probing GitHub on every on-demand sync, because the detection path —
+// unlike the status-sync path — had no freshness guard and never stamped
+// last_checked_at. The frontend re-syncs every 5s while no PR is found, so
+// each repeated sync hit `gh` again and logged a warning. After the fix the
+// first sync probes once and stamps the watch; the immediate second sync is
+// throttled within prSyncFreshnessWindow and must NOT probe again.
+func TestTriggerPRSyncAll_ThrottlesDetectionProbe(t *testing.T) {
+	_, svc, mockClient, store := setupPollerTest(t)
+	ctx := context.Background()
+
+	// Watch with pr_number=0 on a branch that has no PR in the mock — every
+	// FindPRByBranch returns (nil, nil), mirroring "PR not found yet".
+	watch := &PRWatch{
+		SessionID: "s1",
+		TaskID:    "t1",
+		Owner:     "org",
+		Repo:      "missing",
+		PRNumber:  0,
+		Branch:    "feat/never-merged",
+	}
+	if err := store.CreatePRWatch(ctx, watch); err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err := svc.TriggerPRSyncAll(ctx, "t1"); err != nil {
+		t.Fatalf("first TriggerPRSyncAll: %v", err)
+	}
+	if got := mockClient.FindPRByBranchCallCount(); got != 1 {
+		t.Fatalf("expected exactly 1 detection probe on first sync, got %d", got)
+	}
+
+	// Second sync immediately after — well within prSyncFreshnessWindow.
+	if _, err := svc.TriggerPRSyncAll(ctx, "t1"); err != nil {
+		t.Fatalf("second TriggerPRSyncAll: %v", err)
+	}
+	if got := mockClient.FindPRByBranchCallCount(); got != 1 {
+		t.Errorf("expected detection probe to be throttled on second sync (still 1 call), got %d", got)
+	}
+}
+
+// TestTriggerPRDetection_CoalescesConcurrentProbes verifies that parallel
+// syncs for the same watch collapse to a single GitHub probe. The freshness
+// guard alone is racy — concurrent callers can all read a stale
+// last_checked_at and probe simultaneously — so detection runs inside a
+// per-watch singleflight. We force overlap by blocking the probe: the leader
+// enters FindPRByBranch and parks; the followers must coalesce into it rather
+// than issue their own probes.
+//
+// synctest.Wait() supplies the otherwise-unobservable "all followers have
+// joined the singleflight" signal: it returns only once every goroutine is
+// durably blocked (the leader on the release channel, the followers inside
+// singleflight). That makes the coalescing assertion deterministic without a
+// fixed sleep.
+func TestTriggerPRDetection_CoalescesConcurrentProbes(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		_, svc, mockClient, store := setupPollerTest(t)
+		ctx := context.Background()
+
+		watch := &PRWatch{
+			SessionID: "s1",
+			TaskID:    "t1",
+			Owner:     "org",
+			Repo:      "missing",
+			PRNumber:  0,
+			Branch:    "feat/never-merged",
+		}
+		if err := store.CreatePRWatch(ctx, watch); err != nil {
+			t.Fatal(err)
+		}
+
+		const concurrency = 6
+		release := make(chan struct{})
+		mockClient.GateFindPRByBranch(nil, release) // block the leader inside FindPRByBranch
+
+		var wg sync.WaitGroup
+		wg.Add(concurrency)
+		for range concurrency {
+			go func() {
+				defer wg.Done()
+				if _, err := svc.TriggerPRSyncAll(ctx, "t1"); err != nil {
+					t.Errorf("TriggerPRSyncAll: %v", err)
+				}
+			}()
+		}
+
+		// Block until the leader is parked in FindPRByBranch and every other
+		// caller has coalesced into its singleflight call.
+		synctest.Wait()
+		close(release)
+		wg.Wait()
+
+		if got := mockClient.FindPRByBranchCallCount(); got != 1 {
+			t.Errorf("expected concurrent detection probes to coalesce to 1 GitHub call, got %d", got)
+		}
+	})
+}
diff --git a/apps/backend/internal/github/service_pr_watch.go b/apps/backend/internal/github/service_pr_watch.go
index f109cfd384..7bc646f332 100644
--- a/apps/backend/internal/github/service_pr_watch.go
+++ b/apps/backend/internal/github/service_pr_watch.go
@@ -562,7 +562,13 @@ func (s *Service) TriggerPRSyncAll(ctx context.Context, taskID string) ([]*TaskP
 			tp, syncErr = s.triggerPRStatusSync(ctx, w, taskID)
 		}
 		if syncErr != nil {
-			s.logger.Warn("per-repo PR sync failed",
+			// Debug, not Warn: this is best-effort background reconciliation and
+			// the most common failure — a branch with no PR yet, or an
+			// unresolvable repo — is an expected steady state, not an
+			// operational warning. The background poller logs the same failure
+			// at Debug (see poller.detectPRForWatch / checkSinglePRWatch); the
+			// WS handler still returns the error to the caller.
+			s.logger.Debug("per-repo PR sync failed",
 				zap.String("task_id", taskID),
 				zap.String("repository_id", w.RepositoryID),
 				zap.Int("pr_number", w.PRNumber),
@@ -580,7 +586,58 @@ func (s *Service) triggerPRDetection(ctx context.Context, watch *PRWatch, taskID
 	if s.client == nil {
 		return nil, nil
 	}
+	// Coalesce concurrent detection probes for the same watch. Without this the
+	// freshness check below is racy: parallel callers (e.g. the 5s frontend
+	// retry racing the workspace background refresh) can all read a stale
+	// last_checked_at and probe GitHub simultaneously, defeating the throttle.
+	// Keyed distinctly from triggerPRStatusSync's owner/repo/number key so the
+	// two never collide. singleflight only blocks same-key calls, so the nested
+	// triggerPRStatusSync (different key) below cannot deadlock.
+	key := "pr-detect:" + watch.ID
+	v, err, _ := s.syncGroup.Do(key, func() (interface{}, error) {
+		return s.detectPRForWatchOnce(ctx, watch, taskID)
+	})
+	if err != nil {
+		return nil, err
+	}
+	if v == nil {
+		return nil, nil
+	}
+	return v.(*TaskPR), nil
+}
+
+// detectPRForWatchOnce performs a single branch-detection probe for a
+// searching (pr_number=0) watch. It runs inside triggerPRDetection's
+// singleflight so only one probe per watch is in flight at a time.
+func (s *Service) detectPRForWatchOnce(ctx context.Context, watch *PRWatch, taskID string) (*TaskPR, error) {
+	// Throttle branch-detection probes. A watch still searching for its PR
+	// (pr_number=0) has no TaskPR row yet, so triggerPRStatusSync's freshness
+	// check can't gate it — we gate on the watch's own last_checked_at instead.
+	// Without this, a branch whose PR never appears (e.g. an unresolvable repo)
+	// re-hits `gh` on every on-demand sync, and the frontend re-syncs every 5s
+	// while no PR is found — flooding the logs with identical failures.
+	if watch.LastCheckedAt != nil && time.Since(*watch.LastCheckedAt) < prSyncFreshnessWindow {
+		return s.store.GetTaskPRByRepository(ctx, taskID, watch.RepositoryID)
+	}
+
 	pr, err := s.client.FindPRByBranch(ctx, watch.Owner, watch.Repo, watch.Branch)
+	// Stamp last_checked_at regardless of outcome — including on error. The
+	// flood this fixes IS the error path (an unresolvable repo makes `gh` exit
+	// non-zero), so stamping only on success would leave the bug unfixed. The
+	// tradeoff: a transient GitHub error throttles the next on-demand probe for
+	// prSyncFreshnessWindow, which is fine — it stops hammering a failing
+	// endpoint, and the 60s background poller still retries. This diverges
+	// intentionally from poller.detectPRForWatch, which stamps only on success.
+	// The empty-string check/review args clear last_check_status /
+	// last_review_state. That's harmless here — this path only runs for
+	// pr_number=0 (searching) watches, which never carry those fields — and
+	// matches the poller's detection stamp. Only call this on searching
+	// watches; for a resolved PR use the status-sync path instead.
+	now := time.Now().UTC()
+	if tsErr := s.store.UpdatePRWatchTimestamps(ctx, watch.ID, now, nil, "", ""); tsErr != nil {
+		s.logger.Debug("failed to stamp PR watch after detection probe",
+			zap.String("watch_id", watch.ID), zap.Error(tsErr))
+	}
 	if err != nil || pr == nil {
 		return nil, err
 	}